1. Field of the Invention
The present invention relates generally to the tooth structure of earth-boring bits of the rolling cutter variety. More particularly, the present invention relates to improving the wear-resistance of mill- or steel-tooth earth-boring bits.
2. Background Information
The success of rotary drilling enabled the discovery of deep oil and gas reservoirs. The rotary rock bit was an important invention that made that success possible. Only soft formations could be commercially penetrated with the earlier drag bit, but the original rolling-cone rock bit invented by Howard R. Hughes, U.S. Pat. No. 939,759, drilled the hard caprock at the Spindletop Field, near Beaumont, Tex., with relative ease.
That venerable invention, within the first decade of this century, could drill a scant fraction of the depth and speed of the modern rotary rock bit. If the original Hughes bit drilled for hours, the modern bit drills for days. Bits today often drill for miles. Many individual improvements have contributed to the impressive overall improvement in the performance of rock bits.
The early rolling-cone earth-boring bits had teeth formed integrally with the cutters. These bits, commonly known as "steel-tooth" or "mill-tooth" bits, are still in common usage for penetrating relatively soft formations. The strength and fracture-toughness of the steel teeth permits relatively long teeth with long crests, which provide the aggressive gouging and scraping action that is advantageous for the rapid penetration of relatively soft formations.
However, it is rare that a formation interval will consist entirely of soft material with low compressive strength. Often, there are streaks of hard or abrasive materials that a steel-tooth bit must be able to penetrate economically, without damage to the bit.
Although steel teeth possess good strength, their abrasion resistance generally is not adequate to permit rapid penetration of hard or abrasive streaks without damage to the bit. Consequently, it is conventional in the art to provide a layer of wear-resistant material or hard-facing over at least a portion of the teeth of a steel tooth bit. These wear-resistant materials or hard-facings are conventional, and typically consist of particles of tungsten carbide or other hard metal dispersed in a steel or cobalt binder matrix. Such hard-facing materials are applied by melting the binder of the hard-facing material and applying the material over the surfaces of the tooth. The proper application of hard-facing material to steel tooth bits requires considerable skill on the part of the welder.
Internal records at Hughes Christensen Company indicate that the practice of hard-facing steel teeth was initiated in approximately 1929. With the introduction of the tungsten carbide insert (TCI) bit by Hughes Tool Company in the 1950's (see U.S. Pat. No. 2,687,875, Aug. 31, 1954, to Morlan, et al.), the focus of the drilling industry turned to the use of TCI bits. More recently, however, attention again has focused on the improvement and development of earth-boring bits of the mill- or steel-tooth variety because of advances in bearing and seal technology.
It is difficult to apply a relatively thick layer of hard-facing material over the crest or ends of teeth within tolerance.
A tooth with a crest hard-faced to a thickness beyond the tolerance can cause the tooth to interfere with or "strike" an opposing cone. This condition requires expensive and time-consuming grinding of the hard-faced crest to reduce the thickness and eliminate interference. At least as early as 1989, one corner of the steel teeth in one row is beveled to permit application of hard-facing without causing the aforementioned interference between teeth.
U.S. Pat. No. 5,152,194, Oct. 6, 1992, to Keshavan, et al. discloses a method of hard-facing a steel-tooth earth-boring bit, wherein a substantially uniform thickness of hard-facing is provided over the tooth. Each corner of each tooth is rounded to achieve uniform hard-facing thickness. That disclosure does not address the difficulty of applying a thick layer of hard-facing material over a tooth of a steel-tooth earth-boring bit without incurring the problem of tooth strike, which requires costly and time-consuming grinding operations to bring the hard-faced tooth within the clearances and tolerances necessary to avoid strike.
A need exists, therefore, for an earth-boring bit having hard-faced steel tooth structure that permits and facilitates application of hard-facing material in substantial thicknesses over the tooth, while avoiding over-application of hard-facing material.